libmach: many pe handling fixes

- implement windows pread;
- set correct Fhdr.type;
- add ImageBase to all pe "virtual" addresses;
- correct settext parameter order;
- use pclntab/epclntab to find line numbers.

Fixes issue 4841.
Fixes issue 4926.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7405050
path/filepath: better error reporting during TestWinSplitListTestsAreValid

Fixes issue 4930.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7424043